As an experiment, we took some shiny pennies and laid them on a vinegar-soaked paper towel. After a couple of days, they turned quite green on the face-up side.
And stayed shiny on the side that was against the paper towel.
Then we soaked them in olive oil for a couple of days, because I know that’s one way you can clean very old coins. The “green” side looked a little better, although the pennies definitely showed some marking from their journey.
The “shiny” side actually looked a little worse, especially where a couple pennies had overlapped by accident during the vinegar portion of the experiment!
Next time we’ll try lemon juice and salt for a couple days, like we read about at sunnydaytodaymama.